When a plurality of sub-devices is connected to ports linked to a host, the host uses a Universal Serial Bus (USB) selective suspend mode to reduce its power consumption. That is, when a USB device connected to the host stays in an idle mode for a certain time, the host suspends only the port connected to the USB device without affecting operations of the other ports connected to a USB root hub.
When a network device is connected to the host, the host needs to separately install programs such as driver and connection manager for the network device. For example, when a USB network device is connected, the host installs programs such as driver and connection driver, for the USB network device using a separate CD.
As discussed above, to get rid of the inconvenience in separately installing the programs relating to the network device when the network device is connected, a USB composite network device including a plurality of devices is adopted. For example, the USB composite network device includes a USB network device and a CD ROM for automatically installing a driver of the USB network device.
Since the USB composite network device also includes the plurality of the devices, the power consumption can be reduced by applying the USB selective suspend mode to the USB composite network device. However, the USB composite network device may not support the USB selective suspend mode according to its operating system, or it may take much time for the USB device to enter the suspend mode. For example, when the host runs Windows® XP® operating system, the driver of the USB composite network device sends suspend mode transition request information of the devices to the host only when receiving the suspend mode transition request from all of the devices of the USB composite network device. In other words, upon receiving the suspend mode transition request from any one of the devices of the USB composite network device, the driver may not send the suspend mode transition request information of the corresponding device to the host.
For example, when the host runs Windows Vista® operating system, the driver of the USB composite network device suffers from the considerable delay to send the suspend mode request information to the host with respect to a particular device, because of the internal logic for the selective suspend.